Combined cycle or cogeneration plants for generating electric power from solid or liquid fuel generally comprise a combustor for burning the fuel so as to generate steam for a turbine or other use and to heat a fluid for a gas or other fluid turbine. A fluidized bed combustor has advantages for such use. Heretofore, the practice has been to carry out the functions of raising steam and heating the fluid in the same combustor unit. The combustor may be operated at atmospheric pressure, as shown in U.S. Pat. No. 4,116,005, or with a pressurized bed as shown in U.S. Pat. No. 4,223,529. There are advantages in separating or decoupling the fluid heating function of the combustor from its steam raising function, as described in my U.S. Pat. No. 4,406,128, and captioned Combined Cycle Power Plant With Circulating Fluidized Bed Heat Transfer. In the power plant there described the combustor fluidized bed is caused to circulate through a heat exchanger external of the combustor to heat air for a gas turbine. The temperature of the air so heated delivered to the gas turbine is less than that of the combustor fluidized bed. When sulfur-containing fuel is burned in the combustor it is necessary to charge the combustor with sulfur sorbing material, normally limestone particles, along with the fuel to reduce sulfur emission in the flue gas. The optimum temperature range of combustion for this purpose is about 1450.degree.-1700.degree. F. The efficiency of fluid turbines increases materially with increase in temperature of the fluid supplied thereto, and it would be desirable to operate a fluid turbine unit of a combined cycle or cogeneration plant at higher temperatures, if sulfur emissions would not be increased thereby.